The invention relates to an apparatus provided in a pneumatic duct for conveying fiber tufts therein, particularly in the spinning preparation phase and includes a detector for detecting foreign matter, such as metal particles and sparks, and a downstream-arranged deflecting gate which is controlled by the detector and which, when the detector emits a sensing signal, moves pivotally into a position in which it closes the pneumatic duct and at the same time opens a first coupling opening in the pneumatic duct. The first coupling opening leads to a waste container. The apparatus also includes a normally closed second coupling opening leading to the waste container.
In a known apparatus of the above-outlined type two gates are provided: one gate serves for obturating the pneumatic duct and to open or close the first coupling opening, whereas the second gate opens or closes the second coupling opening. The second gate is spring-biased into the closed position so that it opens the second coupling opening by virtue of the pressure which builds up in front of the first gate as a result of the closing of the pneumatic duct by the first gate. The use of two gates renders the device structurally complex and also complicates installation. It is a further disadvantage of the prior art constructions that when the spring bias or the pivotal support of the second gate is interfered with, the latter may remain partially or entirely closed which prevents the separation of the fiber material and may lead to undesired excessive pressures.